Research on imaging apparatuses which irradiate light from a light source (e.g. laser) onto a subject, such as living body, and visualize the information inside the subject, has been energetically progressing in medical fields. An example of such a visualization technology using light is photoacoustic tomography (PAT). A photoacoustic tomography apparatus detects an acoustic wave (typically an ultrasonic wave) generated from a tissue of the living body, which absorbed energy of the light propagating in and diffusing from a subject, at a plurality of locations surrounding the subject. Then the obtained signals are mathematically analyzed, and the information related to the optical property inside the subject, particularly the absorption coefficient distribution, is visualized. Recently pre-clinical research on imaging the blood vessels of small animals using the photoacoustic tomography apparatus, and clinical research on applying the principle of the photoacoustic tomography apparatus to diagnose breast cancer or the like is energetically progressing.
In the case of photoacoustic tomography apparatus and ultrasonic diagnostic apparatus (apparatus for detecting acoustic waves reflected in living body and generating an image) which have been conventionally used in medical fields, images are usually generated using an average acoustic velocity of the subject (sound speed of acoustic wave inside the subject, propagation velocity of acoustic wave inside the subject or propagation speed of acoustic wave inside the subject). Generally sound speed is determined based on an experiential value or document-based values. However propagation speeds have individual differences, and sound speed also changes depending on the method of holding a subject, for example. Therefore if the sound speed used for generating an image and the actual sound speed are different, the resolution of an image drops considerably.
Patent Literature (PTL) 1, for example, discloses a way to solve this problem. According to the technology disclosed in Patent Literature (PTL) 1, sound speed is determined so that brightness or the contrast of each pixel (or voxel) is maximized. Thereby a drop in image quality, due to a mismatch of the sound speed used for generating the image and the actual sound speed, is suppressed.
However in the case of the technology in Patent Literature (PTL) 1, the brightness or contrast of the background noise also increases since the brightness or contrast of each pixel is maximized. Furthermore if noise is included in the detection signals, the sound speed is determined so that the total value of the noise component and normal signal component is maximized, therefore an accurate sound speed cannot be obtained, and the image blurs.    (PTL 1) Japanese Patent Application Laid-Open No. 2000-166925